In flexible disk recorders with interchangeable cartridges sometimes referred to as diskettes, it is necessary to verify that the diskette or cartridge has not been changed by an operator between successive access operations by the using system. In the past, the recorders had a mechanical or electrical sensor that detected the removal of the cartridge from the recorder. The sensor sent an interrupt to the using system if a diskette was removed. Accordingly if an operator exchanged disk cartridges in the middle of an operation, the interrupt would prevent the using system from writing onto an incorrect disk. Writing on an incorrect disk is a serious problem because data overwritten on the incorrect disk is lost.
One difficulty with using mechanical or electrical sensors to detect a cartridge change is that the sensors and their associated electronics are expensive. It is desirable to eliminate this expense when designing a low cost flexible disk recorder.
One technique, to prevent overwriting data on an incorrect disk without using electrical or mechanical sensors, is to do a Volume Identifier (VOLID) check at the beginning of each access to the disk. A VOLID is recorded by a command from the using system on one track of the disk, usually track zero. During each access to the disk, the using system instructs the recorder to move the read/write head to track zero and read the VOLID. If the using system verifies that the VOLID is the expected VOLID, the read or write operation proceeds. If the VOLID read is different from that expected, the system is interrupted and an error is indicated. The difficulty with doing the VOLID check during each access is that the access time to reach a desired data sector is too long. Each access requires the recorder to move the head to track zero to read the VOLID before the head is moved to the track containing the sectors with the desired dataset.